Explosive composition



Rttented Feb. 21, 1950 UNITED STATES 2,498,388 EXPLOSIVE COMPOSITIONAlpheus M. Ball, Blacksburg, Va., assignor, by mesne assignments, to theUnited States of America as represented by the Secretary of War NoDrawing. Application June 8, 1945, Serial No. 598,420

3 Claims.

This invention relates to an improved propellent powder and to animproved method for the preparation of propellent powders. Moreparticularly, the invention relates to an improved method of producingsmokeless powder contain-v ing noncolloidable, nonexplosiveconstituents, and to the improved powdersproduced-thereby.

Powders produced for use in guns must release their useful energy duringthe very short interval in which the projectile is being moved throughthe barrel of the gun. It is necessary, therefore, that these powders becapable of burning rapidly and of developing relatively high pressuresunder normal conditions of use.

Powders produced for use in jet-propulsion devices, such as rockets andthe like, wherein the propulsion is obtained by the rapid escape ofgaseous products of explosion of the powder through a narrow orifice inthe rear of the device, should yield their useful energy in a shorttime, also, as

this type of burning aids in minimizing dispersion of the devices andaccomplishes other desirable effects. However, because of practicallimitations on the weight and resulting strength of the powder chamber,the pressure developed by the jet-propulsion propellent charge must bekept relatively low. Thus, slower-burning forms of powder are requiredfor this use.

An important way of increasing the burning time of smokeless types ofpowder is to mold or extrude or otherwise shape the powder, in colloidedform, into large grains which afiord a relatively low ratio of surface(burning) area per weight of powder.

An important type of propellent powder for use in guns has been thedouble-base nitroglycerin/nitrocellulose powder colloided by the use ofvolatile solvents. The use of these powders, in the form of largegrains, in rockets and the like has not been satisfactory in a number ofrespects. One important fault has been that instead of generating asubstantially constant pressure throughout the interval of burning, thecharges have developed a marked pressure peak of short durationimmediately after ignition. Consequently, the rocket chambers have hadto be built stronger and heavier than otherwise would have beennecessary. Also, it has been observed in firings with the propellentcharges relatively cold, that the burning has progressed in irregularsurges or spurts. The fluctuation in thrust produced thereby has tendedto make the rockets unstable in flight and, consequently, poor inaccuracy.

It has been known to incorporate certain salts 2 in smokeless powders,which are to be used in large caliber guns, in order to reduce the flashproduced at the muzzle end of the gun. In such cases, of course, thepurpose is to prevent ignition of the powder gases as the high pressureis released and combustible material contacts the air. In the case ofmany of the salts used for the purpose of reducing muzzle flash, thedecomposition of the salt during the early stages of the ignitionresults in cavities within the powder grain, thus increasingsubstantially the burning area within the powder and insuring completeburning'of the powder before it reaches the muzzle of the gun. In thecase of jet-propulsion devices the conditions of burning and the likeare quite different,

as has been indicated above. Furthermore, no attempt is made to reducethe flash.

Now, in accordance with thisinvention an improved large grain propellentpowder has been discovered which comprises a noncolloidable,nonexplosive material in a highly subdivided form uniformly incorporatedthroughout the matrix of a double-base smokeless powder colloid. Also,in accordance with this invention a method of producing such propellentpowder has been discovered. 7

It has been found that the hereinbefore-discussed faults ofjet-propulsion powders can be reduced by incorporating in the powder asmall percentage of certain noncolloidable, nonexplosive ingredients,such as potassium nitrate or "potassium sulfate.

The presence of such salts in the propellent increases the burning rateand makes it more uniform. Thus, the initial pressure peak iseliminated, and the weight of the charge which can be used safely in acombustion chamber of a given design is increased. This extends therange of the jet-propulsion device. Also, because the burning is moreuniform, the accuracy of the device is improved. Furthermore, the natureof the burning is otherwise desirably altered, for the temperature belowwhich spurt burning or chuffing occurs isreduced materially.

The salts incorporated in propellent powders to reduce flash have beenof commercial grades having the common powdery or finely-granular,crystalline form. Now, although the incorporation of these salts, in thesame form asused in making flashless smokeless powders, givesadvantageous results in jet-propulsion powders, the resulting finishedpowders are not entirely satisfactory, for they are subject to a type ofheterogeneity shown by the presence of numerous small voids throughoutthe powder matrix. When powder grains containing such voids are burned,the burning successively uncovers the voids, which when reached presentto the flame a surface area greater than that for which the propellentand projectile were designed. The burning rate becomes excessive andirregular, and the flight of the projectile is unpredictable. In theevent the pressure becomes too high the rocket chamber bursts.

Now, in accordance with this invention, an improved powder is made bythe method which, generally described, comprises reducing anoncolloidable, nonexplosive salt 1301a degree of fineness such that atleast 80% by "weight of the salt has a particle mean diameter of lessthan 60 microns, the remainder consisting of particles such that themaximum diameter is less than 80 microns, and uniformly incorporatingthe highly subdivided salt, in small percentage, in the matrix of acolloided nitrocellulose-nitr-oglycerin powder, the salt being mixedwith the nitrocellulose and/or nitroglycerin prior to completelycolloiding the nitrocellulose-nitroglyceri.nmixture.

The method and resulting product produced thereby in accordance withthis invention are illustrated in the following examples, which aregiven for the purposes of illustrating the invention and showing theimprovement therein over the art.

Example 1 As a basis of comparison a batch of double-base smokelesspowder was prepared according to the following procedure: 1345 parts ofnitrocellulose (13.22% N) was wet with 275 parts of 23 alcohol andplaced in a sigma-blade mixer together with 5 parts of diphenylamine, 25parts f ethyl contralite, 125 parts of potassium sulfate of the finenesscommercially supplied and '61 parts additional of 2B alcohol. (All parts"are by weight.) Particle size measurements of the potassium sulfategave these results expressed as per cent by weight: 1%, 0-10 microns;25%, 10-501riicrons; 60%, 50-100 microns; 14%, 100-300 microns. Theingredients were mixed for 5 minutes, then 1000 parts of nitroglycerinand 264 parts of acetone were added. Mixing was continued for 4 hours,after which the powder was extruded in a 4-inch press through aLOGO-0.312 inch die, and'cut into 5.25-inch length grains.

. Within 24 hours after extrusion of the powder, numerous small voids,about 30 per square inch, were seen just beneath the surface of eachgrain. These voids were approximately 1mm. long, and 0.3 mm. indiameter.

Ea'ample 2 A double-base smokeless powder was prepared identical to theone described in Example 1, except that 125 parts of potassium sulfateof the fineness supplied commercially was ground with 1.25 parts of leadstearate (antis'etting agent) for 48 hours in a ball mill prior toincorporation in the powder mix. The particle size of the groundpotassium sulfate was measured as follows: 30% by weight, 0-10 microns;68% by weight, -50 microns; 2% by weight, 50-100 microns.

After extrusion, the powder grains were dried for 2 days in air at20 C.,then for 14'daysin air at '50-55 C. to remove excess solvents. 'No voidsformed during'drying.

Example 3 A double-base smokeless powder was prepared identical to'theo'ne'described in Example 1 except that the 125 parts of potassiumsulfate of the I fineness supplied commerciall was well blended with1.25 parts of lead stearate prior to incorporation in the powder mix.The particle size of the potassium sulfate was of the same distributionas that used in Example 1.

Within 24 hours after extrusion of the powder numerous small voids,about 30 per square inch, were seen just beneath the surface of eachgrain. These Voids were similar in every respect to those obtained inExample 1.

Example 4 A double-base smokeless powder was prepared substantiallyidentical with the powder of Example 2. However, in this example, thepotassium sulfate was prepared by mixing it first in a ball mill with1.25 parts of magnesium carbonate for 1 hour, then grinding it in aBantam Mikro-Pulverizer which was equipped with a 0.027-inch mesh screenand operated at a hammer speed of 16,000 R. P. M. Microscopicexamination of the ground salt indicated that 90% by weight was lessthan 50 microns and the remainder between 50 and microns. The extrudedgrains were dried in air at 20 C. for 2 days and in air at 50 to-55 C.for 14 days. No voids'appeared in the :powder.

Example 5 A smokeless powder was prepared according to Example 4, exceptthat in preparing the finelyground potassium sulfate, zinc oxide wasused in this example instead-of the magnesium carbonate of Example 4.Microscopic examination indicated that by weight, of the potassiumsulfate had a particle size under 50 microns, and the remainder between50 and 80 microns. .No voidsappeared in the finished grains.

Example 6 The following ingredients were incorporated into a double-basesmokeless'pow'der in the manner described in Example 1: 1220 parts ofnitrocellulose (13.22% N), wet with 250 parts of 2B alcohol, 1000 partsof nitroglycerin, '5 parts of diphenylamin'e, 25 parts of ethylcentralite, 264 parts of acetone, 86 parts additional -of 2B alcohol,and 250 parts of potassium sulfate of commercial fineness (particle sizemeasurements same as given in Example I). Mixing time was 3 /2 hours.(All parts areby weight.)

Within 24 hours-after extrusion of the powder, numerous small voids,about per square inch, appeared just beneath the surface of the powdergrains. The voids were approximately 0.5 mm. long and 0.2 mm. indiameter.

Example 7 A smokeless powder was prepared identical to the one describedin Exampleo, except that finely-divided potassium sulfate (see Example 2for method of preparation and "particle size) was used.

Approximately "3 small voids per square inch appeared beneath thesurface ofthegrains within 24 hours after extrusion. No additional voidsappeared later. i

The improvement in the method and product thereof in accordance withthis invention are shown in Examples 2, 4, 5 and 7 when compared withExamples 1, 3, and '6.

The powder in Example 3 is identical with that of Example 1 except forthe addition of about 0.04% by weight of lead stearate. The resultsdemonstrate that the lead stearate alone has -no beneficial effect withregard to the eliminationof voids in the finishedpowder.

The powder of Example 2 contained the same constituents as that ofExample 3, and in the same proportions and having been mixed, colloided,and extruded in the same way. The only difference was that, whereas inExample 3, the potassium sulfate was of the degree of fineness normallyused in making fiashless cannon powder and the resultant powder grainproduced therefrom contained a large concentration of voids ofappreciable dimensions, the powder of Example 2 containedfinely-comminuted potassium sulfate of a much higher degree of finenessand the resultant powder grain produced therefrom was substantially freeof voids. Examples 4 and 5 demonstrated a similar improvement for thesame powder, except that magnesium carbonate and zinc oxide,respectively, were used as the antisetting agents in making thefinely-comminuted salt. The powder of Example '7 demonstrated a similarimprovement over the powder of Example 6, both of them being similar tothose of the other examples except for a smaller proportion ofnitrocellulose and about double the proportion of potassium sulfate, thepotassium sulfate in Example 6 being the same in character as that ofExamples 1 and 3, while that of Example '7 was the same-as that ofExample 2.

Thus, in accordance with this invention, improved solvent-type,large-grain, thick-web. smokeless powders are produced by the method offinely comminuting noncolloidable, nonexplosive ignition-aid salts, suchas potassium sulfate, potassium nitrate, barium nitrate, and the like,preferably in the presence of a minor proportion of an antisettingagent, such as lead stearate, magnesium carbonate, zinc oxide, and thelike, and uniformly incorporating a minor percentage, in the order of5%, of the finely-comminuted material with the powder constituents priorto complete colloiding, and substantially completely colloiding thecolloidable constituents with the aid of volatile solvents to produce,after blocking, pressing and extruding, a large grain smokeless powderwith a finely-comminuted noncolloidable, non-explosive materialuniformly distributed throughout the powder matrix; after drying atlarge grain powder is obtained incorporating a finely-comminutedignition aid and substantially free of voids.

The potassium sulfate used in the improved jet-propulsion powdersprepared in the examples, was such that at least 85% by weight of thematerial was in the particle size range of to 50 microns, and in theremainder no particles had a mean particle diameter greater than 70microns. Potassium sulfate of a fineness satisfactory for use in makingthick-web, solvent-type, jet-propulsion powders should be such that atleast 80% by weight of the material should have a particle size nogreater than about 60 microns and the remainder should containsubstantially no particles larger than 80 microns in diameter.

Although it is preferred to use potassium sulfate, other salts may beutilized in its place to regulate the ignition'characteristics of thepowder. Similar advantages are obtained by reducing the particle size ofsuch salts corresponding to the ranges given above for potassiumsulfate.

The examples given included formulations containing and 10% potassiumsulfate. The amount of salt used will usually be from about 1% to about10%, however, larger amounts may be used, depending upon the purpose andcharacteristics of the salt and the powder.

The salts may be comminuted to the desired particle size either by ballmilling'or grinding, or by any suitable equivalent method. In order toprevent caking of the finely-divided material prior to its use in powdermanufacture, an anti-- setting agent or free-flowing agent, such asmagnesium stearate, lead stearate, magnesium oxide, magnesium carbonate,zinc oxide or similar substances, may be added to the salt ornoncolloidable ingredient before the latter is reduced to the desiredfineness. The percentage of "free-flowing agent may range from about 0to about 5%, but preferably from about 0.5% to about 2.0% by weight ofthe material to be reduced in particle size.

From the available facts, it appears that a plausible explanation forthe improvement in accordance with this invention, is that crystals orcrystal aggregates having a particle size greater than approximately 80microns, when incorporated in colloided smokeless powder containing avolatile solvent, represent points of weakness in the powder whichpermit the colloid to separate and form voids when shrinkagestresses'occur during drying; whereas, crystals or crystal aggregates ofparticle size no greater than about 80 microns mean diameter do notpresent a large enough area of weakness for initiation of materialseparation upon drying.

It will be understood that the advantages of this invention will beobtained when other noncolloidable, nonexplosive ingredients areincorporated in the matrix of a colloided powder, particularly when adrying or solvent removal operation follows the pressing, molding orotherwise shaping, of the powder, provided the noncolloidable,nonexplosive ingredient is sufficiently reduced in particle size so thatthe particles do not provide surfaces of sufficient area to function asnuclei for the initiation of material separation.

This invention is particularly advantageous in the manufacture ofjet-propulsion, solvent-type stick (large web) powder containingpotassium sulfate or potassium nitrate. The powders described in theexamples contained about 40% nitroglycerin, about 48.8% to 53.8%nitrocotton (13.25% N), 5-10% finely-comminuted potassium sulfate andsmall percentages of stabilizers and plasticizer. Such formulations arehighly suitable for use in jet-propelled devices; however, it will beunderstood that other formulations may be used while obtaining theadvantages of this invention. The invention may be applied in bothdouble-base and single-base powders for guns as well as in powders foruse in jet-actuated devices.

Where, in the specification and appended claims, the term thick-web isused in connection with a powder grain, it is meant to designate apowder grain with a web thickness of not less than about 0.20 inch. By"web is meant the shortest distance through the powder betweensubstantially opposite powder surfaces that approach each other duringburning. Where the term 23 alcohol is used in connection with theformulation of a powder it is meant an alcohol having parts by volume ofethyl alcohol, 5 parts by volume of H20, and one-half part by volume ofbenzene.

What I claim and desire to protect by Letters Patent is:

1. A thick-web powder grain of web thickness of not less than about 0.20inch comprising a colloided mixture of nitrocellulose and nitroglycerin,and from about 1% to about 10% by weight of a finely divided saltselected from the group consisting of potassium sulphate, potassiumnitrate and barium nitrate uniformly incorporated therein, at least 80%by weight of said salt having a particle size no greater than about 60microns mean diameter and substantially all of the remainder having aparticle size no greater than about 80 microns mean diameter.

2. A thick-web powder grain of Web thickness of not less than about 0.20inch comprising a colloided mixture of nitrocellulose and nitroglycerin,and from about 1% to about 10% by Weight of finely-comminuted potassiumsulfate uniformly incorporated therein, at least 80% by weight of saidpotassium sulphate having a particle size no greater than about 60microns mean diameter and substantially all of the remainder having aparticle size no greater than about 80 microns mean diameter.

3. A thick-web powder grain of web thickness of not less than about 0.20inch comprising a colloided mixture of nitrocellulose and nitroglycerin,and from about 1% to about 10% by weight of finely-comminuted potassiumnitrate uniformly incorporated therein, at least 80% by Weight of saidpotassium nitrate having a particle size nogreater than about 60 micronsmean (1'- ameter and substantially all of the remainde having a particlesize no greater than about. 8

microns mean diameter.

ALPHEUS M. BALL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. A THICK-WEB POWDER GRAIN OF WEB THICKNESS OF NOT LESS THAN ABOUT 0.20INCH COMPRISING A COLLOIDED MIXTURE OF NITROCELLULOSE AND NITROGLYCERIN,AND FROM ABOUT 1% TO ABOUT 10% BY WEIGHT OF A FINELY DIVIDED SALTSELECTED FROM THE GROUP CONSISTING OF POTASSIUM SULPHATE, POTASSIUMNITRATE AND BARIUM NITRATE UNIFORMLY INCORPORATED THEREIN, AT LEAST 80%BY WEIGHT OF SAID SALT HAVING A PARTICLE SIZE NO GREATER THAN ABOUT 60MICRONS MEAN DIAMETER AND SUBSTANTIALLY ALL OF THE REMAINDER HAVING APARTICLE SIZE NO GREATER THAN ABOUT 80 MICRONS MEAN DIAMETER.